The present invention relates to methods and apparatus for redirecting physical energy, in particular, improved methods and apparatus for forming optical gratings for color displays.
More specifically, the present invention relates to a light diffracting device with several gratings, each with a different pitch, that is suitable for optical display, light coupling, optical recording, light communications, spectral analysis, among others. More particularly, the device can be simply fabricated on standard micro-electronic foundry lines and allows integration between the diffractive structure and circuitry embedded in the semiconductor wafer, while still achieving high diffraction efficiencies (&gt;90%). The diffraction efficiency of the device can also be optionally modulated by an applied field provided by semiconductor circuitry embedded in the substrate.
I. Rectangular Gratings
FIGS. 1a and 1b illustrate a typical transmissive mode grating. FIGS. 1a and 1b include transparent electrodes 5 and 10 coupled to a voltage source 15, grating ridges 20, and liquid crystals 25 between the grating ridges. FIG. 1a also includes incident illumination (incident light) 30.
As is well known, liquid crystals 25 typically are characterized two indices of refraction, n.sub.o, ordinary mode, and n.sub.e, extraordinary mode. In this example, grating ridges 20 have an index of refraction equal to n.sub.e.
In FIG. 1a, when voltage source 15 is not applied to transparent electrodes 5 and 10, "off", liquid crystal 25 and grating ridges 20 have the same index of refraction, n.sub.e. As a result, as illustrated, incident illumination 30 is not diffracted.
In FIG. 1b, when voltage source 15 is applied to transparent electrodes 5 and 10, "on", liquid crystals 25 have an index of refraction equal to n.sub.o. As a result, as illustrated, incident illumination 30 is diffracted according to wavelength of light, as is well known in the art. Three important color wavelengths, red 35, green 40, and blue 45 are shown for convenience.
FIG. 1c illustrates a typical reflective mode grating. FIG. 1c includes a bottom electrode 60, and a masking layer 63.
In contrast to the transmissive mode grating, the bottom electrode 60 is typically manufactured from reflective material.
The grating structure such as that illustrated FIG. 1c, includes parameters such as the width 65 of the grating ridges, the length 70 of one grating period, and duty cycle (width 65 divided by length 70). Such parameters are user controlled and determine the performance of the grating.
In FIG. 1c, masking layer 63 is typically provided to mask undesired colors from being refracted from the device. In this example, primarily green colored light is refracted whereas blue and red colored light is inhibited. The parameters of masking layer 63 are user controlled in conjunction with the grating parameters to control the color of light desired. Masking layer 63 can also be used in conjunction with a diffractive mode device for the same purpose.
With lower spatial frequencies grating structures, light is split between the diffracted orders which survive. Thus, the diffraction efficiency in each single order is reduced.
II. Blazed Gratings
FIG. 2 illustrates a reflective mode blazed grating. FIG. 2 includes electrodes 70 and 75 coupled to a voltage source 80, blazed grating ridges 85, and liquid crystals 90 between the grating ridges. FIG. 2 also includes incident illumination (incident light) 95.
In a transmissive mode grating, typically bottom electrode 75 is typically manufactured from transparent material.
With blazed gratings as illustrated in FIG. 2, parameters such as the height 91 of the blazed grating ridges and the length 92 of one grating period are typical indicators of grating performance. One well known benefit of blazed gratings versus rectangular gratings is that it is more efficient in producing light of selected colors relative to the intensity of the incident illumination for courser frequency gratings. Typically, efficiencies are on the order of 90% of the intensity of the incident illumination.
One drawback with current rectangular grating structure is that for producing an efficient structure, length 92 must be on the order of the wavelength of light of interest. For blazed gratings, courser spatial frequencies are typically used. Current manufacturing techniques for gratings include engraving a substrate with a diamond cutting edge, or embossing a substrate. Further, typical blazed gratings structures formed by current methods are very difficult to fabricate, and are mechanically fragile. Also, it is difficult to apply a uniform electric field to the blazed structure.
III. Related Art
High diffraction efficiency, in a single, non-0.sup.th, diffractive order, is desirable for most applications of diffraction gratings. Fabrication of highly efficient (&gt;90%) diffractive structures is difficult, time consuming and expensive. Holographic fabrication can yield high diffraction efficiencies, but when the gratings of different pitches are required on the same substrate several problems arise. 1) It is difficult to mask off small areas from exposure; 2) It is difficult to balance the diffraction efficiencies of the different gratings to the specified or desired values; and 3) The process is time consuming, and thus expensive in a manufacturing setting.
E. Schulze and W. von Reden, "Diffractive liquid crystal spatial light modulators with optically integrated fine-pitch phase gratings", SPIE, Vol. 2408, pp. 113-119, 1995 and U.S. Pat. Nos. 5,198,912 and 4,970,129 illustrate holographic exposures for polymer dispersed liquid crystal films. A drawback with this approach is that a great deal of light is scattered by these grating devices when no electrical field is applied. Further, the operating voltages of this device are still very high (&gt;100 volts) and thus silicon driving circuitry is not easy to integrate.
Embossed gratings have often been used for inexpensive diffractive structures or refractive structures. One drawback with this approach is that such embossed structures cannot generally be used for both alignment purposes and electrical contact purposes with the substrate circuitry. Further, the physical pressure required for the embossing process often causes brittle semiconductor materials to shatter.
High diffraction efficiencies for visible light with standard fabrication techniques (which produce rectangular grooves) have not been achieved. For example, for visible light, the grating period desired implies 0.25 micron feature sizes, which, at this writing, still imposes very high costs. Diffraction of ultraviolet light with high efficiency, requires even finer-pitch diffractive structures. Such fine periods allow only two diffractive orders (0.sup.th and +1) to survive and thus the light is split between only two possible diffractive orders, thus high diffraction efficiency can be achieved in a single diffractive order as all other diffractive orders in such fine-pitch diffractive structures are extinguished. Further, courser-period rectangular-profile gratings allow several diffractive structures to survive, but the light which is diffracted is split between these diffractive orders, thus lowering the diffraction efficiency attained in any single one diffractive order.
U.S. Pat. Nos. 5,161,059, 4,895,790, 4,846,552, and 5,218,471 discuss methods for fabricated multi-level diffractive structures using micro-electronic techniques and equipment. The methods outlined in these patents generally, however, require 4 photolithographic masks and multiple steps to achieve a fabricated 16 level diffractive structure. Thus, these methods are quite expensive and time consuming.
Thus what is required are improved methods for forming blazed grating structures and Bragg grating structures.